I have recently encountered a problem where importing images from a particular folder results in a never-ending process of building thumbnails. It does not hang, it just never finishes. The progress indicator even moves "backwards" off and on, and attempts to build thumbnails that are already perfectly fine. I can see no problems with any thumbnail that is displayed, but PSU keeps on building... If I try to terminate the process, it starts all over again automatically. I have restored the database from backup and started over, but same result. What is this?

My first thought is that there's a corrupt image in the folder (or an image file that PSU has problems handling). Can you check that?
As it errors out, PSU will retry to build the thumb as it knows that it lacks a thumb for that file. That would explain the up and down progress

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